High speed downlink packet access (HSDPA) has been developed to increase downlink (DL) efficiency and throughput in universal mobile telecommunication system (UMTS) Release 5 (R5) wideband code division multiple access (W-CDMA) systems. The key advantages of HSDPA as compared to UMTS R99/R4 are fast and dynamic link adaptation in the DL and a fast layer 1 hybrid automatic repeat request (H-ARQ). Fast link adaptation is achieved by fast scheduling DL transmissions in a base station, coupled with fast layer 1 DL signaling channels. The signaling channel, a high speed shared control channel (HS-SCCH), conveys radio resource allocation information to a plurality of wireless transmit/receive units (WTRUs).
In frequency division duplex (FDD), an HS-SCCH is sent by means of a spreading factor (SF)=128 channelization code during a three (3) time slot transmission time interval (TTI). The HS-SCCH indicates that data would be transmitted to a WTRU on a high speed downlink shared channel (HS-DSCH) after a particular time offset. The HS-SCCH carries the following information: 1) channelization-code-set information (7 bits); 2) modulation scheme information (1 bit); 3) transport-block size information (6 bits); 4) H-ARQ process information (3 bits); 5) redundancy and constellation version (3 bits); 6) new data indicator (1 bit); and 7) a WTRU identity (16 bits).
The HS-SCCH is sent over three (3) time slots (2 ms TTI), but consists of two (2) fields. Field 1 (first time slot) contains channelization code mapping and modulation format information; and field 2 (second and third time slots) contains transport block size, H-ARQ information, redundancy version and a new data indicator along with a WTRU-specific cyclic redundancy check (CRC).
Alternatively, an enhanced uplink (EU) increases uplink (UL) efficiency and throughput. H-ARQ and Node-B scheduling is part of the EU. Similar to an HSDPA, a new shared DL control channel for EU operation provides fast and dynamic allocation of UL radio resources for UL transmissions. The shared DL control channel for the EU needs to ensure low allocation latencies and efficient radio resources management for UL transmissions. Hereinafter, the shared DL control channel for the purposes of an EU is simply referred to as a UL resource assignment channel.
In order to implement an EU along with an HSDPA, another UL resource assignment channel for the EU could be introduced on top of an existing HS-SCCH for an HSDPA. Thus, it is possible to introduce a separate set of SF=128 DL channels as UL resource assignment channels. With this approach, a WTRU would be required to monitor one or more UL resource assignment channels in addition to the HS-SCCHs for an HSDPA operation. Although this approach is conceptually simple, there are many disadvantages with this scheme, such as WTRU complexity, WTRU battery efficiency, and DL spreading code usage.
Therefore, an efficient EU channel assignment scheme is necessary for supporting both an EU and an HSDPA operation.